The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This disclosure relates to compositions and methods for treating subterranean formations, stimulation treatments in particular.
Stimulation treatments in subterranean wells may be performed to improve the rate at which hydrocarbons (or other valuable resources) are produced, maximize the total amount of hydrocarbons that are produced or both. Such treatments may be divided into two principal categories. Matrix stimulation treatments are those during which fluids are introduced or pumped into the well at rates and pressures that do not fracture the rock surrounding the wellbore. Fracturing treatments are those during which fluids are introduced or pumped into the well at rates and pressures that cause the rock surrounding the wellbore to fracture. The fractures may then propagate away from the wellbore and increase the surface area of rock available to the wellbore by orders of magnitude.
Fluids employed during stimulation treatments may contain acids that may dissolve and increase the permeability of producing formations. Hydrochloric acid may be used to stimulate carbonate formations, while hydrofluoric acid may be used to treat sandstone formations. Other organic acids such as formic acid and acetic acid may also have application.
Carbonates are sedimentary rocks deposited in a marine environment and are largely of biological origin. The nature of local deposition contributes to their extensive heterogeneity across all scales—from pores, to grains and to textures. Carbonate rocks may exhibit a complex texture and pore network resulting from their depositional history and subsequent diagenesis.
Matrix acidizing and fracture acidizing are conventionally used to release hydrocarbons in such formations, but the stimulation may be impeded by a lack of heterogeneity of carbonate surfaces. Surface heterogeneity favors the creation of a more complex network of conductive channels (or “wormholes”) as the acid penetrates the formation, and may maximize production efficiency.
Hydrochloric acid (HCl) at a 15 wt % concentration, accompanied by a suitable level of corrosion inhibitor to reduce corrosion to completion and metallic tools, may be used in acid stimulation operations. The principal chemical reaction is shown in Eq. 1.2HCl+CaCO3→CaCl2+CO2+H2O  (Eq. 1)
A number of difficulties may arise due to the high sensitivity of the entire carbonate formation, which by nature may be highly uneven in permeability distribution, to a strong mineral acid system. Notably, etching of the formation by the acid may be confined to locations that are already permeable. Highly permeable areas attract more acid flow due to decreased diffusion limitations, which in turn creates larger wormholes that consume more acid, leaving low-permeability areas untouched. Therefore, the network of wormholes may not be optimal and acid may be unnecessarily wasted.
Conventional acidizing fluids, such as hydrochloric acid or a mixture of hydrofluoric and hydrochloric acids, have high acid strength and may react quickly with fines and scale nearest the wellbore, and may have a tendency to corrode tubing, casing and downhole equipment, such as gravel pack screens and downhole pumps, especially at elevated temperatures. In addition, above 200° F. (93° C.), straight HCl is not recommended in some cases because of its destructive effect on the rock matrix. Due to the type of metallurgy, long acid contact times and high acid sensitivity of the formations, scale removal with hydrochloric acid and hydrochloric acid mixtures has been largely unsuccessful.